The present invention relates to an arrayed waveguide grating (AWG), and the manufacture and use of such a grating. In particular, the present invention relates to the geometry of an AWG that is suitable for, but not exclusively for providing a thermal operation.
Optical systems increasingly use wavelength division multiplexing (WDM), in which a number of distinct optical signals are transmitted at different wavelengths, generally down an optical fibre. For example, optical communication in the so called xe2x80x9cCxe2x80x9d band may use 40 channels, all frequencies spaced apart by regular intervals. One optical signal can be transmitted at each frequency down a single optical fibre. There are the possibilities, for example, that 56 channels may be used in the xe2x80x9cLxe2x80x9d band.
A key component in WDM systems is the demultiplexer for separating the optical signals at a plurality of wavelengths into the individual channels at individual wavelengths. This may be done by using a splitter and a number of different filters tuned to the individual frequencies, by components that demultiplex the light directly, or by a combination of these components.
One approach to filtering and demultiplexing is an arrayed waveguide grating (AWG), also known as a phased-array device. The operation and design of AWGs is described, for example, in xe2x80x9cPHASAR-based WDM-devices; principles, design and applicationsxe2x80x9d, Meint K. Smit, IEEE Journal of Selected Topics in Quantum Electronics Vol. 2, June 1996.
FIG. 1 illustrates a conventional AWG device. The device includes an array 11 of waveguides 3 arranged side by side on a substrate 1 and extending between an input star coupler 13 and an output star coupler 15. The input and output star couplers 13, 15 may be defined by a wide core region in which light can travel freely in the two-dimensional plane of the substrate. This region is known as the free propagation region. Input 17 and output 19 optical waveguides are divided to input light into the array 11 of waveguides and to output light respectively. There may in particular be a plurality of input waveguides 17 and/or output waveguides 19.
As an example, FIG. 2 illustrates the output star coupler of a system with a plurality of output waveguides. The ends 21 of the array waveguides 11 are usually on a geometric circle 23 of radius r whose centre is at centre 25 of an image plane 27. The output waveguides 19 are arranged on the image plane, which also constitutes a circle. Note that the centres of the circles are not coincident, and need not have equal radii.
The lengths of the individual waveguides 3 of the array 11 differ (see FIG. 1), and the shapes of the star couplers 13, 15 are chosen so that light input on the input optical waveguides 17 passes through the array of waveguides and creates a diffraction pattern on the output waveguide or waveguides, such that light of a predetermined central wavelength creates a central interference peak at the centre 25 of the image plane. Light with frequencies slightly higher or lower than the predetermined central frequency is imaged with a central interference peak slightly above or below the centre of the image plane.
In order to achieve this result the optical path length difference between adjacent waveguides of the array is chosen so that it is an integral multiple of the central wavelength. Thus light at the central wavelengths which enters the array of waveguides in phase will also leave in phase and will create the central diffraction spot at the centre of the image plane. Light with a slightly different frequency will arrive at the output star coupler with slight phase differences across the array, which will cause the light to be imaged to a spot on the image plane a little further away from the central spot.
Thus the plurality of output waveguides arranged on the output plane receive light of slightly different frequencies. Equally spaced output waveguides correspond to equally spaced frequencies, to a first order of approximation.
FIG. 2 shows the use of one or more output waveguides connected to the output star coupler 15. It is alternatively or additionally possible to arrange a plurality of input waveguides on the input star coupler with the same effect. Equally, it will be appreciated that the terms input and output are merely used for convenience, as such a device is in fact bi-directional and reciprocal in its performance.
It will be appreciated that the lengths of the individual waveguides in the array 11 are critical to the performance of the AWG. If the difference in length between the optical paths change, then so will the transmission wavelengths of the AWG filter. The optical path length is the physical path length multiplied by the refractive index of the material.
AWG filters are hence inherently temperature sensitive on account of the temperature sensitivity of the refractive index of the waveguide material and, to a lesser extent, due to the expansion coefficient of the material. For instance, in a silica-based planar waveguide device, the wavelength at the centre of the filter passband typically increases by about 0.01 nm (nanometer) per degree Celsius. In other words, for each 10 degree Celsius rise in temperature, the centre wavelength will increase by 0.1 nanometers.
Due to the decreasing wavelength spacing between adjacent optical channels (as WDM systems have developed into DWDM systems), it is desirable to have a filter which is not sensitive to changes in temperature.
Various solutions have been proposed to prevent such AWGs being affected by variations in ambient temperature.
For instance, the use of a heater or a Peltier element attached to the AWG to ensure that the AWG temperature remains at a predetermined, preferred value. Such an arrangement requires control circuitry, as well as consumes power.
It is therefore preferable that the operation of the AWG is itself largely independent of temperature i.e. the AWG operation is athermal.
xe2x80x9cOptical Phased Array Filter Module With Passively Compensated Temperature Dependencexe2x80x9d by G Heise et al, ECOC 98, describes one manner of stabilising such filters passively against thermal drifts. The proposed solution is to utilise a modified input coupling section whereby the input waveguide 17 is attached to a compensating rod. As the temperature changes, the thermal expansion of the compensating rod shifts the physical position of the input 17 so as to retune the filter to compensate for the temperature drift of the phased array chip. This mechanical movement of the position of the input 17 will be subject to effects such as vibration and fatigue.
European Patent application no. EP 0 919 840 A1 by Inoue et al describes an alternative athermal optical waveguide device, in which a groove is formed across the array 11 of waveguides. The groove is then filled with a material having a temperature co-efficient of refractive index of different sign from that of the temperature co-efficient of refractive index of the waveguide. For example, the groove is typically filled with silicone, which has a refractive index temperature co-efficient of xe2x88x9230 x that of silica (the typical material used to form the waveguides). Thus by using silicone filled slots which vary in length between the array guides by about {fraction (1/30)}th of the path length difference between the individual array guides, the AWG filter becomes temperature insensitive. The centre frequencies of the filter pass bands are therefore also temperature insensitive.
However, the main problem with this approach is that the diffraction loss across these slots is relatively significant. This is due to the fact that the optical energy is not guided by a waveguide across the slot.
xe2x80x9cAthermal and Center Wavelength Adjustable Arrayed-Waveguide Gratingxe2x80x9d by K Maru et al, OFC 2000, describes an alternative manner of providing an athermal AWG. The proposed AWG has several trenches with a crescent shape formed in the input star coupler 13. Such trenches (or slots) are tilled with silicone, such that the temperature dependent propagation phase change in the waveguide array is cancelled out. Compared with the Inoue patent application, this structure has advantages that the diffraction loss is significantly reduced. This is principally due to the fact that the diffraction loss from the trench is then confined in direction to that normal to the substrate, since the optical waves propagating through the star coupler are already and intentionally diffracting freely in the plane of the waveguide. The diffraction across a slot in the arrayed waveguides by comparison is in two dimensions and hence leads to a larger loss penalty. However, there are limits to how much the loss can be reduced using this approach as diffraction loss in the direction normal to the substrate can still occur.
It is therefore an object of the present invention to provide an arrayed waveguide that substantially addresses one or more other problems of the prior art.
According to a first aspect, the present invention provides an optical waveguide device for guiding an optical signal comprising a substrate, a waveguide extending at least partially across the substrate, and at least two slots extending transversely across the waveguide, wherein the separation between the slots, in the direction of propagation of the optical signal in the waveguide, is substantially       (                  n        ⁢                  xe2x80x83                ⁢        Δ        ⁢                  xe2x80x83                ⁢        λ            2        )    ,
where n is an odd integer and xcex94xcex the difference in the wavelength of the radiation and guided modes of the optical signal resolved in the direction of propagation.
In other words the radiation generated at one slot can be viewed as destructively interfering with that at the next slot, or as constructively interfering with the guided wave after the next slot. Alternatively the phase advance of the radiation fields compared with the guided mode introduces a component of converging wavefront that partially focuses the radiation onto the guide at the far side of the next slot.
Slots provide no waveguide action and allow free diffraction to occur. By choosing the separation of the slots so that the radiation lost from one slot (or groove) interferes destructively with that from the next slot, this results in some of the radiation from the first slot being effectively coupled back into the waveguide at the next slot. This reduces the total insertion loss of the slots. It will be appreciated that an optical signal may comprise a variety of wavelengths. In the preferred embodiment, the separation between the slots is defined by the centre wavelength of the optical signal, although it will be appreciated that the invention will work with any wavelength in, or close to, the range of wavelengths contained within the optical signal.
The loss reduction obtained by selecting the optimum gap spacing is maintained within 0.01 dB of the minimum for a wavelength variation of about 50 nm from the optimum design value       (                  n        ⁢                  xe2x80x83                ⁢        Δ        ⁢                  xe2x80x83                ⁢        λ            2        )    .
This implies that this loss reduction technique would benefit from a filter operating over the whole of the xe2x80x98Lxe2x80x99 or xe2x80x98Cxe2x80x99 band in a WDM transmission system. However, the effect is still seen to be significant for a wavelength (xcex94xcex) variation of up to xc2x1100 nm from the optimum value of xcex94xcex.
Preferably the slot separation is the separation between the respective side of each slot upon which the optical signal is first incident. For optimum performance, it is preferable that the separation between the start of each slot is       (                  n        ⁢                  xe2x80x83                ⁢        Δ        ⁢                  xe2x80x83                ⁢        λ            2        )    .
This is because the field of the optical signal diffracts outwardly from the slot as soon as it leaves the waveguiding region.
Preferably the device comprises at least two waveguides extending at least partially across the substrate. The invention may be applied to waveguide devices containing one or more waveguides. In such instances, as the wavelength of the optical signal being transmitted along the respective waveguides may differ slightly, then the separation between the slots in any respective waveguide may only be an approximation of       (                  n        ⁢                  xe2x80x83                ⁢        Δ        ⁢                  xe2x80x83                ⁢        λ            2        )    ,
in order to ease manufacture.
Preferably wherein said slots extend transversely across said at least two waveguides. In order to provide different thicknesses of slot at the intersection of a slot with each respective waveguide, the slot geometry may be wedge shaped, or indeed any appropriate shape.
Preferably the device comprises at least three of said slots each extending transversely across the waveguide, wherein the pitch between the slots in the direction of propagation of the optical signal is substantially       (                  n        ⁢                  xe2x80x83                ⁢        Δ        ⁢                  xe2x80x83                ⁢        λ            2        )    .
The diffraction loss across a slot increases superlinearly with slot width and so it is preferable to use numerous small slots in preference to one or a couple of large slots. By spacing apart these slots as per the present invention, then losses are further decreased.
Preferably the device comprises an array of waveguides extending at least partially across the substrate. Such an array may be used for a variety of optical processing functions such as, optical multiplexing, demultiplexing, or dispersion compensation.
Preferably the length of the slots in the direction of propagation is a function of the optical length of the respective waveguide across which the slots transversely extends.
Preferably the device comprises an arrayed waveguide grating (AWG), the AWG further comprising a first coupling region and a second coupling region, with the array of waveguides extending between the two coupling regions.
Preferably the slots are less than 10 microns in length in the direction of propagation of the optical signal. Slots of approximately 10 microns or less in width have been shown to be substantially less lossy than larger size slots. Alternatively, slots can be of 5 microns or less in width, with such slots being even less lossy per unit length than 10 micron slots. The width of the slot used is likely to be set by the technology used for their fabrication.
Preferably wherein said slots contain a different material from that forming the waveguide. For instance, the different material can have a temperature coefficient of refractive index of different sign to that of the material forming the waveguide, such as silicone.
Alternatively, the different material may have electro-optic properties, or indeed any optical properties arranged to change the optical signal passing along the guide in a controllable manner.
A second aspect of the present invention provides a method of manufacturing an optical waveguide device for guiding an optical signal, the method comprising steps of: forming a waveguide in a substrate, waveguide extending at least partially across the substrate; and forming at least two slots extending transversely across the waveguide, such that the separation between the two slots, in the direction that an optical signal would normally propagate along the waveguide, is substantially       (                  n        ⁢                  xe2x80x83                ⁢        Δ        ⁢                  xe2x80x83                ⁢        λ            2        )    ,
where n is an odd integer and xcex94xcex the difference in wavelength of the radiation and guided modes of the optical signal resolved in the direction of propagation. Of course, various alternative manufacturing processes can be utilised to implement this method, as will be apparent to a skilled person. For instance, waveguide layers can be deposited using PECVD (plasma enhanced chemical vapour deposition). After deposition the layers can be consolidated in an annealing process, and the slots etched using RIE (reactive ion etching).
A third aspect of the present invention provides an optical apparatus for processing an optical signal, the apparatus comprising at least one input for receiving an optical signal; at least one output for providing an optical signal for onward transmission; and an optical waveguide device for guiding an optical signal comprising a substrate, a waveguide extending at least partially across the substrate, and at least two slots extending transversely across the waveguide, wherein the separation between the slots, in the direction of propagation of the optical signal in the waveguide, is substantially       (                  n        ⁢                  xe2x80x83                ⁢        Δ        ⁢                  xe2x80x83                ⁢        λ            2        )    ,
where n is an odd integer and xcex94xcex the difference in wavelength of the radiation and guided modes of the optical signal resolved in the direction of propagation.
A fourth aspect of the present invention provides a node for a telecommunications network, the node being arranged to transmit and receive a telecommunication signal, and the node comprising at least one optical waveguide device for guiding an optical signal comprising a substrate, a waveguide extending at least partially across the substrate, and at least two slots extending transversely across the waveguide, wherein the separation between the slots, in the direction of propagation of the optical signal in the waveguide, is substantially       (                  n        ⁢                  xe2x80x83                ⁢        Δ        ⁢                  xe2x80x83                ⁢        λ            2        )    ,
where n is an odd integer and xcex94xcex the difference in wavelength of the radiation and guided modes of the optical signal resolved in the direction of propagation. A final aspect of the present invention provides a method of processing an optical signal, the method comprising receiving an input optical signal; processing the optical signal; outputting at least a portion of the processed optical signal, wherein the processing step includes transmitting the optical signal along an optical waveguide device, the waveguide device comprising a substrate, a waveguide extending at least partially across the substrate, and at least two slots extending transversely across the waveguide, wherein the separation between the slots, in the direction of propagation of the optical signal in the waveguide, is substantially       (                  n        ⁢                  xe2x80x83                ⁢        Δ        ⁢                  xe2x80x83                ⁢        λ            2        )    ,
where n is an odd integer and xcex94xcex the difference in wavelength of the radiation and guided modes of the optical signal resolved in the direction of propagation.